Islet transplantation is the most specific therapy for beta cell replacement and achievement of glucose homeostasis. However, despite a T cell-directed immunotherapy, majority of islet allografts, succumb to rejection~ typically co-incident with the production of donor specific IgG alloantibodies, which in addition to being poor prognostic clinical indicators are thought to exert a pathogenic role in vivo. Our preliminary studies in a murine model indicated a requisite role of B-cell antigen presentation in activation o alloreactive CD4 T lymphocytes. Therefore, our contention is that the induction of robust transplantation tolerance will require unresponsiveness at the level of both the B- and T-cell compartments. As such, we performed a preclinical trial of islet transplantation in Cynomolgus macaques utilizing an induction immunotherapy regimen, which included a CD20 specific B cell depleting agent. The results of this trial indicated that transient B cell depletion at the time of transplantation protects islet allografts from rejection for a significant length of time. However, donor-specific IgG alloantibodies were eventually produced in the majority of the recipients, coincident with the loss of islet graft function. Therefore, the main mechanistic proposition of th present application is that establishment of B cell tolerance is required for achievement of immunological tolerance to islet allografts. Our goal is to develop a clinically feasible, B cell-directed immunotherapeutic strategy based on the homeostatic mechanisms governing the development of B cell tolerance to self-antigens. To this end, we aim to induce donor-specific B cell tolerance by targeting the key regulatory pathway of B cell survival, life-span and selection: the TNF-related cytokine known as BLyS/BAFF. It was recently recognized that this cytokine regulates antigen mediated negative selection of newly emerging transitional B cells and, thereby, serves as the major micro-environmental cue responsible for shaping the mature B cell repertoire. Here, we hypothesize that limiting the availability of systemic BLyS/BAFF during B cell compartment reconstitution in the presence of an islet allograft will promote a sustained state of B cell tolerance. We will test this clinically pertinent concept in the setting of islet alo-transplantation in Cynomolgus monkeys.